1. Field of the Invention
The present invention relates to shape memory polymers and, more particularly, to a shape memory foam characterized by two temporary shapes and one permanent shapes.
2. Description of the Related Art
Shape memory polymers (SMPs) are a class of smart materials that can be designed to undergo programmed changes in shape and stiffness in response to an external stimulus, such as heat, light, magnetic field, and pH. Thermally triggered SMPs are the most extensively employed, and have been used in a wide range of applications including smart textiles and apparel, biomedical devices, and deployable aircraft structures. The shape-changing functionality of SMPs is achieved by memorization of a permanent shape through physical or chemical crosslinks, and programming of an arbitrary temporary shape by deformation and subsequent immobilization of the polymer chains. Upon mobilization of the polymer chains through application of a stimulus, such as heat, the SMP object changes shape from the programmed temporary state to the memorized permanent state, driven by entropic elasticity.
Recently, shape memory functionality has been married with the high compressibility and low density of porous materials to develop SMP foams. While some studies have reported on closed-cell SMP foams, most studies to-date have focused on open-cell foams based on polyurethane or epoxy chemistries. These foams are well-suited for applications where large expansion ratios are required from a light-weight, compact packaged state. For instance, Solowski and colleagues developed cold-hibernated elastic memory foams (CHEM) for deployable space aircraft structures. These polyurethane-based SMP foams had high full/stowed volume ratios of up to 40 and were capable of being stored in the cold hibernated condition for over 1 year. SMP epoxy-based foams with increased electrical conductivity and large compressibility have also been reported.
SMP foams have also recently received increased attention for biomedical applications, such as the use of CHEM polyurethane-based foams as an occlusive material for embolization of aneurysms. It was found that CHEM foams were able to successfully occlude internal maxillary arteries in a dog model, although some residual necks and recurrences were reported. A laser-triggered SMP foam was also developed as an aneurysm occlusion device. There, an SMP comprised of hexamethylene diisocyanate, N,N,N′,N′-tetrakis(2-hydroxypropyl)eythlenediamine, and triethanolamine with a glass transition temperature (Tg) of 45° C. was placed in a PDMS basilar-necked aneurysm model and deployed by laser activation. The SMP foam was able to fully expand within 60 s of activation. With recent developments in polymer systems with triggering temperatures near body temperature, SMP foams with potential for studies under physiological conditions have also been enabled. For instance, a poly(ε-caprolactone)-co-poly(ethylene glycol) SMP foam capable of expanding once hydrated at 37° C. can be used for cell mechanobiology studies.
A limitation of current SMP foams is the inability to prescribe both the programmed and recovered shapes, as the recovered shape is restricted to the permanent, as-fabricated shape of the foam. Triple shape memory polymers (TSMPs) offer one way to overcome this limitation, where control over two temporary shapes can be achieved. As opposed to conventional SMPs that feature one transition temperature, TSMPs possessing two separate transition temperatures enable programming of two independent temporary shapes. TSMPs can be programmed to undergo two controlled shape changes, from temporary shape 1 to temporary shape 2, and from temporary shape 2 back to the permanent shape. This approach could be used to control both the programmed shape of an SMP foam (temporary shape 1) and the deployed state upon recovery (temporary shape 2).
Researchers have developed several approaches to fabricate TSMPs. The first triple shape memory effect was reported in two different polymer network systems where each had two distinct transition temperatures. TSMPs have also been developed using bilayer systems, where two epoxy dual-shape polymers with Tg of 38 and 75° C. were bonded together to create a bimorph. Other approaches to TSMP fabrication and triggering have also recently been reported, such as a TSMP hydrogel that employed dipole-dipole interactions to achieve triple shape behavior. A supermolecular composite containing a SMP polyurethane and cholesteryl isonicotinate has been developed where the hydrogen bonding between the carboxyl groups of the polyurethane and the pyridine ring of the cholesteryl isonicotinate enable triple shape memory. Recent advances have also resulted in the development of TSMPs triggered through alternating magnetic fields rather than thermal triggering, as well as TSMPs with reversible actuation. The present invention have also developed a new approach to creating TSMPs by fabricating a composite system where poly(ε-caprolactone) electrospun fibers were embedded in an epoxy matrix. In that approach, triple shape memory was achieved through the two distinct transition temperatures: melt transition of the poly(ε-caprolactone) fibers and glass transition of the epoxy matrix. These composite-based triple shape polymers was referred to as triple shape memory composites (TSMCs).
One approach to easily fabricate TSMCs is to use polymerization induced phase separation (PIPS). There, two TSMC systems were fabricated differing in the nature of their fixing mechanisms. The first was a polypropylene glycol-epoxy/poly(ε-caprolactone) where the epoxy and poly(ε-caprolactone) begin as a miscible blend when mixed 80° C. and phase separate upon curing of the epoxy phase. This system achieved triple shape memory through a lower Tg of the epoxy phase and a separate higher melt transition temperature of the poly(ε-caprolactone) phase. The second system incorporated a poly(ethylene oxide)-based epoxy rather than the polypropylene glycol-epoxy, and triple shape memory behavior was achieved through two separate melt transition temperatures of the epoxy and poly(ε-caprolactone) phases. Both materials exhibited good shape memory fixing and recovery of two temporary shapes, but with vastly different stiffness at room temperature and water sensitivity.
However, no one has been able to fabricate triple shape shame memory foams, which offer the potential for low density materials that can be triggered to deploy with a large volume change, unlike their solid counterparts that do so at near-constant volume. While examples of shape memory foams have been reported in the past, they have been limited to dual SMPs, i.e., polymers that feature one switching transition between an arbitrarily programmed shape and a single permanent shape established by constituent crosslinks.